| The frequency synthesizer in Bluetooth communications system is mainly used generate the local oscillatioan frequency for the mixer and upconversion&downconversion. Therefore, not only does the frequency synthesizer provide a stable and accurate frequency but also the other parameters of frequency synthesizer,for example,output frequency phase noise,lock time, have a great impact on system performance.In this paper, at the beginning, both the principle and the block of the charge pump frequency synthesizer are introduced.Then,The purpose of this paper is research and design 2.4GHz, 79-channel Delta-Sigma Fractional-N Frequency Synthesizer used in the Bluetooth.In this design, a PFD employs the TSPC DFF, the PFD structure is simple but with high-speed, high-performance; Charge pump uses differential circuit structure for good matching between the MOS and better noise suppression of the charge pump; The charge pump employs regulated cascade circuit structure for reducing current mismatch; The using 3-order passive RC filter, it can effectively filter out high-level noise;the using of LC-VCO, it has low phase noise; the fractional divider using multi-modulus divider (MMD) working in a appropriate ratio.In the paper, a 3-order MASH 1-1-1 (Multi-stAge noise SHaping)Delta-Sigma modulator is used to randomize the instantaneous division ratio of the MMD.MASH1-1-1 modulator not only has good noise shaping performance, but also has a fine frequency resolution and lower phase noise and rapid switching frequency .Based on TSMC 0.18um CMOS process, the circuit was simulated by Cadence Spectre and the system was simulated by ADS.The frequency synthesizer with a supply voltage of 1.8v,has a frequency resolution of 1220Hz and a loop bandwidth of 100kHz.The measured phase noise is -90dBc/Hz at offset frequency of 100 kHz and the measured stable time is about 40 us for a frequency of jump 80MHz. The Frequency Synthesizer is satisfied with the requirements of Bluetooth transceiver. |
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